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2022 Vol. 39, No. 2

2022-2 Contents
2022, 39(2): 1-1.
Editorial Notes
Preface to the Special Issue: Climate Change and Variability of Tropical Cyclone Activity
Liguang WU, Bin WANG, Johnny C. L. CHAN, Kyung-Ja HA, Il-Ju MOON, Jun MATSUMOTO, Zhemin TAN, Ke FAN
2022, 39(2): 203-204. doi: 10.1007/s00376-021-1020-3
Understanding of the Effect of Climate Change on Tropical Cyclone Intensity: A Review
Liguang WU, Haikun ZHAO, Chao WANG, Jian CAO, Jia LIANG
2022, 39(2): 205-221. doi: 10.1007/s00376-021-1026-x
The effect of climate change on tropical cyclone intensity has been an important scientific issue for a few decades. Although theory and modeling suggest the intensification of tropical cyclones in a warming climate, there are uncertainties in the assessed and projected responses of tropical cyclone intensity to climate change. While a few comprehensive reviews have already provided an assessment of the effect of climate change on tropical cyclone activity including tropical cyclone intensity, this review focuses mainly on the understanding of the effect of climate change on basin-wide tropical cyclone intensity, including indices for basin-wide tropical cyclone intensity, historical datasets used for intensity trend detection, environmental control of tropical cyclone intensity, detection and simulation of tropical cyclone intensity change, and some issues on the assessment of the effect of climate change on tropical cyclone intensity. In addition to the uncertainty in the historical datasets, intertwined natural variabilities, the considerable model bias in the projected large-scale environment, and poorly simulated inner-core structures of tropical cyclones, it is suggested that factors controlling the basin-wide intensity can be different from individual tropical cyclones since the assessment of the effect of climate change treats tropical cyclones in a basin as a whole.
Original Paper
Growing Threat of Rapidly-Intensifying Tropical Cyclones in East Asia
Kin Sik LIU, Johnny C. L. CHAN
2022, 39(2): 222-234. doi: 10.1007/s00376-021-1126-7
This study examines the long-term change in the threat of landfalling tropical cyclones (TCs) in East Asia over the period 1975–2020 with a focus on rapidly intensifying (RI) TCs. The increase in the annual number of RI-TCs over the western North Pacific and the northwestward shift of their genesis location lead to an increasing trend in the annual number of landfalling RI-TCs along the coast of East Asia. The annual power dissipation index (PDI), a measure of the destructive potential of RI-TCs at landfall, also shows a significant increasing trend due to increases in the annual frequency and mean landfall intensity of landfalling RI-TCs. The increase in mean landfall intensity is related to a higher lifetime maximum intensity (LMI) and the LMI location of the landfalling RI-TCs being closer to the coast. The increase in the annual PDI of East Asia is mainly associated with landfalling TCs in the southern (the Philippines, South China, and Vietnam) and northern parts (Japan and the Korean Peninsula) of East Asia due to long-term changes in vertical wind shear and TC heat potential. The former leads to a northwestward shift of favorable environments for TC genesis and intensification, resulting in the northwestward shift in the genesis, RI, and LMI locations of RI-TCs. The latter provides more heat energy from the ocean for TC intensification, increasing its chances to undergo RI.
Impact of the Western Pacific Tropical Easterly Jet on Tropical Cyclone Genesis Frequency over the Western North Pacific
Ruifen ZHAN, Yuqing WANG, Yihui DING
2022, 39(2): 235-248. doi: 10.1007/s00376-021-1103-1
Although it is well known that the tropical easterly jet (TEJ) has a significant impact on summer weather and climate over India and Africa, whether the TEJ exerts an important impact on tropical cyclone (TC) activity over the western North Pacific (WNP) remains unknown. In this study, we examined the impact of the TEJ on the interannual variability of TC genesis frequency over the WNP in the TC season (June−September) during 1980−2020. The results show a significant positive correlation between TC genesis frequency over the WNP and the jet intensity in the entrance region of the TEJ over the tropical western Pacific (in brief WP_TEJ), with a correlation coefficient as high as 0.66. The intensified WP_TEJ results in strong ageostrophic northerly winds in the entrance region and thus upper-level divergence to the north of the jet axis over the main TC genesis region in the WNP. This would lead to an increase in upward motion in the troposphere with enhanced low-level convergence, which are the most important factors to the increases in low-level vorticity, mid-level humidity and low-level eddy kinetic energy, and the decreases in sea level pressure and vertical wind shear in the region. All these changes are favorable for TC genesis over the WNP and vice versa. Further analyses indicate that the interannual variability of the WP_TEJ intensity is likely to be linked to the local diabatic heating over the Indian Ocean-western Pacific and the central Pacific El Niño-Southern Oscillation.
Tropical Cyclones over the Western North Pacific Strengthen the East Asia–Pacific Pattern during Summer
Sining LING, Riyu LU
2022, 39(2): 249-259. doi: 10.1007/s00376-021-1171-2
The contribution of tropical cyclones (TCs) to the East Asia–Pacific (EAP) teleconnection pattern during summer was investigated using the best track data of the Joint Typhoon Warning Center and NCEP-2 reanalysis datasets from 1979 to 2018. The results showed that the TCs over the western North Pacific (WNP) correspond to a strengthened EAP pattern: During the summers of strong convection over the tropical WNP, TC days correspond to a stronger cyclonic circulation anomaly over the WNP in the lower troposphere, an enhanced seesaw pattern of negative and positive geopotential height anomalies over the subtropical WNP and midlatitude East Asia in the middle troposphere, and a more northward shift of the East Asian westerly jet in the upper troposphere. Further analyses indicated that two types of TCs with distinctly different tracks, i.e., westward-moving TCs and northward-moving TCs, both favor the EAP pattern. The present results imply that TCs over the WNP, as extreme weather, can contribute significantly to summer-mean climate anomalies over the WNP and East Asia.
Quantifying the Contribution of Track Changes to Interannual Variations of North Atlantic Intense Hurricanes
Jun LU, Liguang WU, Shunwu ZHOU
2022, 39(2): 260-271. doi: 10.1007/s00376-021-1116-9
Previous studies have linked interannual variability of tropical cyclone (TC) intensity in the North Atlantic basin (NA) to Sahelian rainfall, vertical shear of the environmental flow, and relative sea surface temperature (SST). In this study, the contribution of TC track changes to the interannual variations of intense hurricane activity in the North Atlantic basin is evaluated through numerical experiments. It is found that that observed interannual variations of the frequency of intense hurricanes during the period 1958–2017 are dynamically consistent with changes in the large-scale ocean/atmosphere environment. Track changes can account for ~50% of the interannual variability of intense hurricanes, while no significant difference is found for individual environmental parameters between active and inactive years. The only significant difference between active and inactive years is in the duration of TC intensification in the region east of 60°W. The duration increase is not due to the slow-down of TC translation. In active years, a southeastward shift of the formation location in the region east of 60°W causes TCs to take a westward prevailing track, which allows TCs to have a longer opportunity for intensification. On the other hand, most TCs in inactive years take a recurving track, leading to a shorter duration of intensification. This study suggests that the influence of track changes should be considered to understand the basin-wide intensity changes in the North Atlantic basin on the interannual time scale.
Changes in Typhoon Regional Heavy Precipitation Events over China from 1960 to 2018
Yangmei TIAN, John L. MCBRIDE, Fumin REN, Guoping LI, Tian FENG
2022, 39(2): 272-283. doi: 10.1007/s00376-021-1015-0
In earlier studies, objective techniques have been used to determine the contribution of tropical cyclones to precipitation (TCP) in a region, where the Tropical cyclone Precipitation Event (TPE) and the Regional Heavy Precipitation Events (RHPEs) are defined and investigated. In this study, TPE and RHPEs are combined to determine the Typhoon Regional Heavy Precipitation Events (TRHPEs), which is employed to evaluate the contribution of tropical cyclones to regional extreme precipitation events. Based on the Objective Identification Technique for Regional Extreme Events (OITREE) and the Objective Synoptic Analysis Technique (OSAT) to define TPE, temporal and spatial overlap indices are developed to identify the combined events as TRHPE. With daily precipitation data and TC best-track data over the western North Pacific from 1960 to 2018, 86 TRHPEs have been identified. TRHPEs contribute as much as 20% of the RHPEs, but 100% of events with extreme individual precipitation intensities. The major TRHPEs continued for approximately a week after tropical cyclone landfall, indicating a role of post landfall precipitation. The frequency and extreme intensity of TRHPEs display increasing trends, consistent with an observed positive trend in the mean intensity of TPEs as measured by the number of daily station precipitation observations exceeding 100 mm and 250 mm. More frequent landfalling Southeast and South China TCs induced more serious impacts in coastal areas in the Southeast and the South during 1990−2018 than 1960−89. The roles of cyclone translation speed and “shifts” in cyclone tracks are examined as possible explanations for the temporal trends.
Projection of the Future Changes in Tropical Cyclone Activity Affecting East Asia over the Western North Pacific Based on Multi-RegCM4 Simulations
Jie WU, Xuejie GAO, Yingmo ZHU, Ying SHI, Filippo GIORGI
2022, 39(2): 284-303. doi: 10.1007/s00376-021-0286-9
Future changes in tropical cyclone (TC) activity over the western North Pacific (WNP) under the representative concentration pathway RCP4.5 are investigated based on a set of 21st century climate change simulations over East Asia with the regional climate model RegCM4 driven by five global models. The RegCM4 reproduces the major features of the observed TC activity over the region in the present-day period of 1986−2005, although with the underestimation of the number of TC genesis and intensity. A low number of TCs making landfall over China is also simulated. By the end of the 21st century (2079−98), the annual mean frequency of TC genesis and occurrence is projected to increase over the WNP by 16% and 10%, respectively. The increase in frequency of TC occurrence is in good agreement among the simulations, with the largest increase over the ocean surrounding Taiwan Island and to the south of Japan. The TCs tend to be stronger in the future compared to the present-day period of 1986−2005, with a large increase in the frequency of strong TCs. In addition, more TCs landings are projected over most of the China coast, with an increase of ~18% over the whole Chinese territory.
Northward Shift in Landfall Locations of Tropical Cyclones over the Western North Pacific during the Last Four Decades
Ting CHEN, Shumin CHEN, Mingsen ZHOU, Chaoyong TU, Aoqi ZHANG, Yilun CHEN, Weibiao LI
2022, 39(2): 304-319. doi: 10.1007/s00376-021-1077-z
This study analyzes landfall locations of tropical cyclones (TCs) over the western North Pacific during 1979–2018. Results demonstrate that the landfall locations of TCs over this region have shifted northward during the last four decades, primarily due to the shift of landfalling TC tracks, with the decreasing/increasing proportion of westward/northward TC tracks. In particular, the northward shift of the landfalling TCs was not related to their formation locations, which have not markedly changed, whereas “no-landed” TCs have significantly shifted northward. TC movement was significantly and positively correlated to the zonal component of the steering flow, while the correlation between TC movement and the meridional component of the steering flow was relatively unobvious. The westward steering flow in the tropical central Pacific that occurred around the formation and early development of the westward TCs was significantly weakened, which was unfavorable for their westward movement, thereby, causing the higher proportions of northward moving tracks. This weakened westward flow was related to the northward shift of the subtropical high ridge, which was caused by significant weakening of the southern part of the subtropical high. The vertical wind shear, sea surface temperature, and convective available potential energy also showed that the northern region of the western North Pacific became more favorable for TC development, whereas the upper divergence, low-layer relative vorticity, and accumulated water vapor content were not obviously related to the northward shift of TCs.
Climatology of Tropical Cyclone Extreme Rainfall over China from 1960 to 2019
Ying LI, Dajun ZHAO
2022, 39(2): 320-332. doi: 10.1007/s00376-021-1080-4
Tropical cyclone extreme rainfall (TCER) causes devastating floods and severe damage in China and it is therefore important to determine its long-term climatological distribution for both disaster prevention and operational forecasting. Based on the tropical cyclone (TC) best-track dataset and TC precipitation data from 1960 to 2019, the spatiotemporal distribution of TCER affecting China is analyzed.   Results show that there were large regional differences in the threshold for TCER in China, decreasing from the southeastern coast to the northwest inland. TCER occurred infrequently in northern China but had a high intensity and was highly localized. The frequency and intensity of TCER showed slightly increasing trends over time and was most likely to occur in August (41.0%). Most of the TC precipitation processes with extreme rainfall lasted for four to six days, with TCER mainly occurring on the third to fourth days. TCER with wide areas showed a northwestward prevailing track and a westward prevailing track.   Strong TCs are not always accompanied by extreme precipitation while some weak TCs can lead to very extreme rainfall. A total of 64.7% (35.3%) of the TCER samples occurred when the TC was centered over the land (sea). TCER≥250 mm was located within 3° of the center of the TC. When the center of the TC was located over the sea (land), the extreme rainfall over land was most likely to appear on its northwestern (northeastern) side with a dispersed (concentrated) distribution. TCER has unique climatic characteristics relative to the TC precipitation.
A Climatological Perspective on Extratropical Synoptic-Scale Transient Eddy Activity Response to Western Pacific Tropical Cyclones
Yao HA, Zhong ZHONG, Haikun ZHAO, Yimin ZHU, Yao YAO, Yijia HU
2022, 39(2): 333-343. doi: 10.1007/s00376-021-0375-9
An observational study focusing on the contribution of tropical cyclones (TCs) that form over the western North Pacific (WNP) to the synoptic-scale transient eddy activity (STEA) over the North Pacific during the boreal autumn and early winter in the period 1979–2019 is presented in this paper. Statistical results show that WNP TCs entering the mid-latitudinal North Pacific provide significant positive effects on the pentad mean strength of STEA, which is primarily concentrated over the Kuroshio/Oyashio Extensions (KOE) and regions from east of Japan to 160°W in the lower and mid-to-upper troposphere, respectively. TC intensity is highly indicative of the subsequent STEA with a correlation coefficient of 0.37/0.33/0.45 at 300 hPa/500 hPa/850 hPa exceeding the 99% confidence level for the period 1979–2019. The strength of STEA in the upper troposphere associated with TCs presents a more significant linear growth with TC intensity than that at the mid-to-lower levels after the cyclones enter the KOE region, suggesting that the impact of TCs on STEA gradually increases with height. Further analyses reveal that the contribution of TCs accounts for 4%–6% of the total STEA change over the KOE region during the late autumn and early winter. In addition, the influence of TCs on STEA experienced an interdecadal decrease from the early 2000s through the early 2010s.
Letters & Notes
What Drives the Decadal Variability of Global Tropical Storm Days from 1965 to 2019?
Yifei DAI, Bin WANG, Weiyi SUN
2022, 39(2): 344-353. doi: 10.1007/s00376-021-0354-1
The tropical storm day (TSD) is a combined measure of genesis and lifespan. It reflects tropical cyclone (TC) overall activity, yet its variability has rarely been studied, especially globally. Here we show that the global total TSDs exhibit pronounced interannual (3−6 years) and decadal (10 years) variations over the past five-to-six decades without a significant trend. The leading modes of the interannual and decadal variability of global TSD feature similar patterns in the western Pacific and Atlantic, but different patterns in the Eastern Pacific and the Southern Indian Ocean. The interannual and decadal leading modes are primarily linked to El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), respectively. The TSDs-ENSO relationship has been steady during the entire 55-year period, but the TSDs-PDO relationship has experienced a breakdown in the 1980s. We find that the decadal variation of TSD in the Pacific is associated with the PDO sea surface temperature (SST) anomalies in the tropical eastern Pacific (PDO-E), while that in the Atlantic and the Indian Ocean is associated with the PDO SST anomalies in the western Pacific (PDO-W). However, the PDO-E and PDO-W SST anomalies are poorly coupled in the 1980s, and this “destructive PDO” pattern results in a breakdown of the TSDs-PDO relationship. The results here have an important implication for seasonal to decadal predictions of global TSD.